The mid-infrared (MIR, about 2 to 20 μm) spectral band is particularly useful for molecular sensing applications due to the excitation of fundamental rotational and vibrational transitions allowing for sensitive and selective detection of molecules in both gas and liquid phases. For liquid sensing, attenuated total reflection (ATR) and evanescent field absorption measurements facilitate probing of samples, which are too opaque for transmission absorption measurements such as in environmental monitoring, process analysis, and biological applications.
If total internal reflection (TIR) occurs, an evanescent field extends at the interface between the optically denser waveguide (refractive index n1) and an optically thinner adjacent medium (refractive index n2; with n1>n2). The penetration depth, dp, of the evanescent field is commonly defined as:dp=λ/2π(n12 sin2θ−n22)1/2,where λ is the wavelength of the radiation, and θ is the incoupling angle. Absorbing species present within the penetration depth of the evanescent field interact with radiation, resulting in attenuation of the frequencies where resonant energy transfer to the vibrational modes of molecules occurs.
As the intensity of the evanescent field strongly depends on the waveguide geometry, decreasing the fiber diameter or tapering a section of the fiber increases the intensity of the evanescent field, and thereby improves the sensitivity to absorbing species at or close to the waveguide surface. Evanescent field absorption measurements follow a pseudo Lambert-Beer relationship, where the absorbance A is defined as:A=(εCl)r where ε is the molar absorptivity, C is the concentration, l is the optical path length, and r is the fraction of power guided outside the waveguide core. Maximum evanescent field intensity, and therefore a maximum value of r, would occur in a single mode waveguide, which is thickness-matched to the emission frequency of a corresponding laser light source. Ideal optimization conditions are limited to mono-mode laser light sources providing a platform for highly integrated MIR evanescent field sensing systems.
Laser light sources provide enhanced spectral density compared to conventional broadband MIR sources. Consequently, if a narrow emission band is matched to a carefully selected absorption band characteristic for the analyte of interest, sufficient selectivity but higher sensitivity can be achieved in a more compact and miniaturizable sensing system.
Quantum cascade lasers (QCLs) have successfully been applied in sensing formats due to their wide coverage of MIR emission frequencies. Distributed feedback (DFB) QCLs provide narrow emission line-width (approximately 0.006 cm−1) by incorporating a grating exposed or buried at the surface of the laser ridge, thus facilitating selection of overlapping analyte bands.